High dielectric constant (HDC) materials have many microelectronic applications, such as DRAMs, embedded DRAMs, SRAMs, FeRAMS, on-chip capacitors and high frequency capacitors. Typically, these applications employ HDC materials in a capacitive structure, although the present invention may be used to make an BDC thin film with improved properties which is not part of a capacitor.
To facilitate construction of larger DRAMs with correspondingly smaller memory cells, capacitor structures and materials which can store the necessary charge in smaller spaces are needed. One of the most promising avenues of research to achieve this goal is the area of HDC materials. HDC materials have dielectric constants of greater than about 50. Examples of particular HDC materials are metal oxide materials such as, lead zirconate titanate (PZT), barium titanate (BaTiO.sub.3), strontium titanate (SrTiO.sub.3), and barium strontium titanate (BST). It is desirable that such a material, if used for DRAMs and other microelectronics applications, be formable over an electrode and underlying structure (without significant harm to either), have low leakage current characteristics and long lifetime, and, for most applications, possess a high dielectric constant. The present invention relates to a method of forming a HDC film, for example, a BST dielectric film, with improved sidewall stoichiometry.
While BST materials have been manufactured in bulk form previously, the physical and electrical properties of the material is not well understood when BST is formed as a thin film (generally less than 5 um) on a semiconducting device. Methods to form the (Ba,Sr) TiO.sub.3 material include deposition by a metal organic chemical vapor deposition (MOCVD) process using appropriate precursors. Typical MOCVD deposition of BST utilizes the precursors of Ba(bis(2,2,2,6-tetramethyl-3,5-heptanedionate)).sub.2 tetraethylene glycol dimethyl ether; Sr(bis(2,2,2,6-tetramethyl-3,5-heptanedionate)) tetraethylene glycol dimethyl ether and Ti(bis(isopropoxy)).sub.2 bis(2,2,2,6-tetramethyl-3,5-heptanedionate).sub.2. A liquid delivery system mixed, metered and transported the precursors at room temperature and high pressure to a heated zone, where the precursors were then flash vaporized and mixed with a carrier gas, typically argon, to produce a controlled temperature, low pressure vapor stream. The gas stream was then flowed into a reactor mixing manifold where the gas stream mixed with oxidizer gases. Typically the oxidizer gases were O.sub.2 and N.sub.2 O. The mixture of the gas stream and the oxidizer gases then passed through a shower head injector into a deposition chamber. In the MOCVD deposition, both the ratio of the concentrations of the metalorganic compounds in the vaporized liquid and the deposition conditions determine the final film stoichiometry. However, the MOCVD BST deposition process suffers from the inhomogeneity in stoichiometry (A:B site ratio) on 3-D structures.
In addition, in submicron microcircuits such as DRAM capacitors, particular constraints are placed on BST thin film. First, the annealing temperature for BST thin films must generally be kept far below the temperatures commonly used for sintering bulk BST ceramics (generally less than 700.degree. C. vs. typically greater than 1100.degree. C. for bulk BST) to avoid damage to the underlying device structure. Thus, the grain nucleation and growth kinetics of the BST crystal lattice is inhibited resulting in smaller grain sizes. Second, the desired film thickness in microelectronic applications may be much less than 5 um (preferably between about 0.05 um and about 0.1 um). It has been found that median grains sizes generally less than half the BST film thickness are required to control dielectric uniformity and avoid shorted capacitors. Finally, when a BST film is formed in a microelectronic application such as a container or a stud, the sidewall components of the film generally contains less titanium than is present in the horizontal components of the container or stud formation. The percentage of titanium in the film is critical to the physical end electrical functionality of the film. It has been shown that the titanium must be between about 50% to about 53.5% of the BST film in order for the film to have beneficial physical and electrical properties. Thus, a method for producing a HDC material such as BST in a thin film structure having good dielectric properties and uniform titanium content is needed.